among the young or adult Crustacea are there forms which might resemble the maggots of the Diptera or Hymenoptera, the larvæ of the Coleoptera, or the caterpillars of the Lepidoptera, still less any bearing even a distant resemblance to the quiescent pupæ of these animals. The pupæ, indeed, cannot at all be regarded as members of an original developmental series, the individual stages of which represent permanent ancestral states, for an animal like the mouthless and footless pupa of the Silkworm, enclosed by a thick cocoon, can never have formed the final, sexually mature state of an Arthropod. In the development of the Insecta we never see new segments added to those already present in the youngest larvæ, but we do see segments which were distinct in the larva afterwards become fused together or disappear. Considering the parallelism which prevails throughout organic nature between palæontological and embryonic development, it is therefore improbable that the oldest Insects should have possessed fewer segments than some of their descendants. But the larva of the Coleoptera, Lepidoptera, etc., never have more than nine abdominal segments, it is therefore not probable that they represent the original young form of the oldest Insects, and that the Orthoptera, with an abdomen of eleven segments, should have been subsequently developed from them. Taking into consideration on the one hand these difficulties, and on the other the arguments which indicate the Orthoptera as the order most nearly approaching the primitive form, it is my opinion that the “incomplete metamorphosis” of the Orthoptera is the primitive one, _inherited_ from the original parents of all Insects, and the “complete metamorphosis” of the Coleoptera, Diptera, etc., a subsequently _acquired_ one. CHAPTER XII. PROGRESS OF EVOLUTION IN CRUSTACEA.